Go to Lowtown at night to get started, you'll need to fight off a pack of thieves as soon as you get there. Follow you minimap to find Sister Petrice's safehouse. Do her quest, it'll basically involve a bunch of battles. Keep in mind to have your rogue disarm the traps you'll encounter, it's a good way to earn XP even if they aren't in your way.

At the end when you've exited the caverns you'll get to either turn the mage over to the Qunari or fight for his freedom. It's better to refuse to hand over Saarebas since you'll get the Qunari mage's help and a Talisman of Saarebas. Fighting for him will also earn you points with your companions. Return to Sister Petrice and get seven Sovereigns for the quest.